The present invention relates to a device and a method for wireless transferring of information on physiological variable values, and especially of such information that has been determined by invasive measurements.
There is a general need for invasive measurements of physiological variables. For example, when investigating cardiovascular diseases it is strongly desired to obtain local measurements of pressure and flow in order to evaluate the condition of the subject.
Therefore, methods and devices have been developed for disposing a miniature sensor at the location where the measurements should be performed, and for communicating with the miniature sensor.
For example, a system and a method for measuring fluid pressure of a living body is described in U.S. Pat. No. 3,853,117. A sensor for implantation in the cranial cavity is formed as a mechanically resonant structure, the resonant frequency being a function of the fluid pressure. By applying sonic energy from an external source, and by receiving the responding resonance signal, it is possible it is possible to detect the resonance frequency, and consequently to determine the pressure of the fluid.
Another example of a known intracranial pressure monitor is known through U.S. Pat. No. 4,026,276, wherein is described an apparatus including a passive resonant circuit having a natural frequency influenced by ambient pressure. The local pressure is measured by observation of the frequency at which energy is absorbed from an imposed electromagnetic field located externally of the cranium.
In order to communicate the measured representation of the physiological variable devices based on acoustical as well as electromechanical interaction have been developed. In both cases, the sensor comprises a resonance element, its resonance frequency being a function of the physiological variable to be determined. Energy is radiated towards the resonance element from an external transmitter of acoustical or electromagnetic waves, respectively. The frequency of the transmitted energy is swept over a pre-selected range, and is registered by a monitoring unit. During the frequency sweep the registering unit will detect the resonance frequency of the resonance element, since a drop of the monitored transmitted energy will occur at this frequency.
Both of the examples above of known devices for invasive measurements of physiological variables are examples of passive systems, i.e. the sensor inside the body does not require a source of energy, such as a battery or electricity provided via electrical leads.
For guiding a sensor to a specific point of measurement during investigating cardiovascular diseases it is known to mount a miniature sensor at the distal end of a guide wire or a catheter. The guide wire or the catheter is inserted into a blood vessel such as the femoral artery, and is guided by fluoroscopy to local sites within the cardiovascular system where improper functioning is suspected.
The development of miniature sensors, or micro-sensors, for a number of physiological variables, including pressure, flow, temperature etc., constitutes a historical landmark.
However, the assembly of the sensor and the associated cables and connectors is difficult to perform in a cost-efficient manner due to the small physical dimensions, the required mechanical precision and uncompromisable demands on patient safety. More specifically, it is estimated that about half the cost, or more, of the total manufacturing cost for such devices are traceable to connectors and cables.
As a consequence, devices performing these functions are still expensive, and the spread of their use is limited to areas of highest clinical priority. The cost aspect is further emphasised by the fact that devices for invasive procedures must be regarded as disposable items, due to the risk of transmitting infectious diseases. If the cost of cables and connectors could be minimised or even eliminated, large savings would be possible.
Another problem with passive sensors of the kind disclosed in U.S. Pat. No. 4,026,276 is the undesired electromagnetic coupling between the transmitter/receiver in the one hand, and the sensor on the other hand. This coupling is due to the fact that the power supply and the signal transmission are not functionally separated. A manifestation of this problem is that the output signal of the system is influenced by the position of the sensor, which obviously is an undesired property.
This problem could be overcome by adding active electronic circuitry to the sensor, including a local transmitter operating at a frequency other than the frequency used for providing electric power to the sensor and the circuitry. Thereby, the function of wireless power supply should be separated from that of signal transmission and, consequently, the output signal should not be influenced by the position of the sensor. Such a solution has been described by R. Puers, xe2x80x9cLinking sensors with telemetry: Impact on the system designxe2x80x9d, Proc. 8th Int. Conf. Solid State Sensors and Actuators, Transducers-95, Stockholm Sweden, Jun. 25-29, 1995, Vol 1, pp 47-50. However, a drawback of this solution is that it is difficult to miniaturise to the size desired for medical use with a guide wire. Furthermore, wideband systems of this kind are amenable to electromagnetic interference and disturbances.
Thus, there is a need for an improved communication system for communication with a sensor positioned inside a body of a subject for invasive measurement of a physiological variable, said communication system exhibiting reduced sensitivity to the position of the sensor as well as to electromagnetic interference.
It is the object of the present invention to provide a device for overcoming the problem referred to above.
This object is achieved with a passive biotelemetry system according to claim 1 of the appended claims.
According to the invention, an electronic circuit forming a transponder unit is provided that may be integrated on a single silicon die of extremely small dimensions, and requires only a small number of discrete components which all can be accommodated and packaged within the available space of a guide wire having an outside diameter of 0.4 mm, or on a separate plate for implantation. Alternatively, the transponder unit could be inserted into a living body as an implant.
The system operates at a low bandwidth and hence is not sensitive to electromagnetic disturbances. Neither is it sensitive to position, nor to the precise control of the transmission properties of the medium.
Furthermore, it eliminates the requirement of cables and connectors to connect the sensor with the environment outside of the body.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention are given by way of illustration only. Various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.